1. Field of the Invention
This invention relates generally to structures produced by techniques of nanotechnology, and methods of producing such structures.
More specifically, the invention relates to such structures and devices incorporating at least one element, essentially in one-dimensional form, which is of nanometer dimensions in its width or diameter, which is produced with the aid of a catalytic particle, and which is commonly termed a “nanowhisker.”
The invention relates also to a method of forming a nanowhisker of a certain material on a substrate of a dissimilar material.
2. Brief Description of the Prior Art
Nanotechnology covers various fields, including that of nanoengineering, which may be regarded as the practice of engineering on the nanoscale. This may result in structures ranging in size from small devices of atomic dimensions, to much larger scale structures—for example, on the microscopic scale. Typically, nanostructures are devices having at least two dimensions less than about 1 μm (i.e., nanometer dimensions). Ordinarily, layered structures or stock materials having one or more layers with a thickness less than 1 μm are not considered to be nanostructures. Thus, the term nanostructures includes free-standing or isolated structures that have two dimensions less than about 1 μm, that have functions and utilities different from those of larger structures, and that are typically manufactured by methods different from conventional procedures for preparing somewhat larger, i.e., microscale, structures. Although the exact boundaries of the class of nanostructures are not defined by a particular numerical size limit, the term has come to signify such a class that is readily recognized by those skilled in the art. In many cases, an upper limit of the size of the at least two dimensions that characterize nanostructures is about 500 nm. In some technical contexts, the term “nanostructure” is construed to cover structures having at least two dimensions of about 100 nm or less. In a given context, the skilled practitioner will recognize the range of sizes intended. In this application, the term “nanostructure” is broadly intended to refer to an elongated structure having at least two transverse dimensions less than about 1 μm, as indicated above. In more preferred applications, such dimensions will be less than about 100 nm, more preferably less than about 50 nm, and even more preferably less than about 20 nm.
Nanostructures include one-dimensional nanoelements, essentially in one-dimensional form, that are of nanometer dimensions in their width or diameter, and that are commonly known as nanowhiskers, nanorods, nanowires, nanotubes, etc.
The basic process of whisker formation on substrates by the so-called VLS (vapor-liquid-solid) mechanism is well known. A particle of a catalytic material, usually gold, is placed on a substrate and heated in the presence of certain gases to form a melt. A pillar forms under the melt, and the melt rises up on top of the pillar. The result is a whisker of a desired material with the solidified particle melt positioned on top. See Wagner, Whisker Technology, Wiley, N.Y., 1970, and E. I Givargizov, Current Topics in Materials Science, Vol. 1, pages 79-145, North Holland Publishing Company, 1978. In early applications of this technique, the dimensions of such whiskers were in the micrometer range, but the technique has since also been applied for the formation of nanowhiskers. For example, International Patent Application Publication No. WO 01/84238 (the entirety of which is incorporated herein by reference) discloses in FIGS. 15 and 16 a method of forming nanowhiskers, wherein nanometer sized particles from an aerosol are deposited on a substrate and these particles are used as seeds to create filaments or nanowhiskers.
Although the growth of nanowhiskers catalyzed by the presence of a catalytic particle at the tip of the growing whisker has conventionally been referred to as the VLS (Vapor-Liquid-Solid process), it has come to be recognized that the catalytic particle may not have to be in the liquid state to function as an effective catalyst for whisker growth. At least some evidence suggests that material for forming the whisker can reach the particle-whisker interface and contribute to the growing whisker even if the catalytic particle is at a temperature below its melting point and presumably in the solid state. Under such conditions, the growth material, e.g., atoms that are added to the tip of the whisker as it grows, may be able to diffuse through a the body of a solid catalytic particle or may even diffuse along the surface of the solid catalytic particle to the growing tip of the whisker at the growing temperature. Persson et al., “Solid-phase diffusion mechanism for GaAs nanowires growth,” Nature Materials, Vol. 3, October 2004, pages 687-681, shows that, for semiconductor compound nanowhiskers there may occur solid-phase diffusion mechanism of a single component (Ga) of a compound (GaAs) through a catalytic particle. Evidently, the overall effect is the same, i.e., elongation of the whisker catalyzed by the catalytic particle, whatever the exact mechanism may be under particular circumstances of temperature, catalytic particle composition, intended composition of the whisker, or other conditions relevant to whisker growth. For purposes of this application, the term “VLS process,” or “VLS mechanism,” or equivalent terminology, is intended to include all such catalyzed procedures wherein nanowhisker growth is catalyzed by a particle, liquid or solid, in contact with the growing tip of the nanowhisker.
For the purposes of this specification the term “nanowhisker” is intended to mean a one-dimensional nanoelement with a width or diameter (or, generally, a cross-dimension) of nanometer size, the element preferably having been formed by the so-called VLS mechanism, as defined above. Nanowhiskers are also referred to in the art as “nanowires” or, in context, simply as “whiskers” or “wires.”
Several experimental studies on the growth of nanowhiskers have been made, the most important reported by Hiruma et al. They grew III-V nanowhiskers on III-V substrates in a metal organic chemical vapor deposition (MOCVD) growth system. See Hiruma et al., J. Appl. Phys. 74, page 3162 (1993); Hiruma et al., J. Appl. Phys. 77, page 447 (1995); Hiruma et al., IEICE Trans. Electron. E77C, page 1420 (1994); Hiruma et al., J. Crystal Growth, 163, pages 226-231 (1996).
More recently, growth of Si nanowires on Si substrates has been demonstrated. See, e.g., Westwater et al., J. Vac. Sci. Technol., B 1997, 15, page 554. Very recently, growth of Ge nanowires on Si substrates was also demonstrated. See Kamins et al., Nano Lett. 2004, 4, pages 503-506, Web published Jan. 23, 2004.
In the prior art in general, many different approaches have been tried in order to realize perfect epitaxial growth of III-V materials on silicon substrates. The primary motivation for these strong efforts is that, if such a technology could be developed, a very wide spectrum of so called III-V heterostructure devices may be incorporated with main-stream silicon technology, thus opening the way to highly advanced high-speed and opto-electronic devices incorporated with silicon.
Besides the efforts toward integrating III-V materials on Si, other approaches toward the specific goal of efficient light-emission using Si have been proposed—for example, the formation of porous Si via electrochemical etching (Canham, L. T., Appl. Phys. Lett., 1990, 57, page 1046) and the incorporation of luminescent defects, such as rare-earth impurities (Michel et al, Semiconduct. Semimet., 1998, 49, page 111.
Epitaxial growth of III-V semiconductors on Si presents a number of difficulties, such as lattice mismatch, differences in crystal structure (III-V's have a polar zinc blende or wurtzite structure whereas Si has a covalent diamond structure), and a large difference in thermal expansion coefficient. Much work has been done on planar growth of layers of III-V materials on Si substrates using different approaches such as buffer layers, growth on patterned Si surfaces, and selected area growth from small openings. See, for example, Kawanami, H., Sol. Energy Mater. Sol. Cells, 2001, 66, page 479.
A major challenge has been to avoid the formation of antiphase domains related to the initiation of III-V growth on two atomic planes of silicon differing by one atomic layer, which leads to the formation of anti-phase domain walls and defective material. In Ohlsson et al., “Anti-domain-free GaP, grown in atomically flat (001) Si sub-μm-sized openings”, Appl. Phys. Lett., Vol 80, No. 24, 17 Jun. 2002, pages 4546-4548, to address the problem of antiphase domains, GaP nanocrystals were grown on a Si(001) substrate surface through openings in a mask of SiO2. The mask openings were defined by e-beam lithography. Etching and chemical stripping followed, and organic residues were removed by oxygen plasma. A high annealing temperature of 1000° C. was used to remove Si oxide and to provide atomic flatness on the silicon surface on which GaP is nucleated. An atomically flat surface is a surface that presents a single crystal facet and does not exhibit atomic steps.
I prior U.S. patent application Ser. No. 10/613,071, published as No. 2004-0075464, to Samuelson et al., and International Patent Application Publication No. WO-A-04/004927 (both of which publications are incorporated herein by reference), there are disclosed methods of forming nanowhiskers by a chemical beam epitaxy method. Nanowhiskers are disclosed having segments of different materials, with abrupt or sharp heterojunctions therebetween. Structures are disclosed comprising nanowhiskers of, for example, gallium arsenide extending from a silicon substrate. Processes are disclosed for forming epitaxial layers of III-V materials on a silicon substrate, involving initial formation of nanowhiskers on the substrate and using the nanowhiskers as nucleation centers for an epitaxial layer.
Improvements are desirable in the formation of nanowhiskers of III-V materials (or having at least a base portion of III-V material) on a substrate of Group IV material to ensure that the nanowhiskers are grown in a highly reliable way with accurate predetermined dimensions and structure, and with accurately predetermined physical characteristics for implementing the above structures and processes. More generally, improvements are desirable in the formation of nanowhiskers having at least a base portion of a predetermined material on a substrate of a dissimilar material.